There is described in the prior art several methods for breaking down calcium sulfate in an effort to obtain lime and sulfur oxides.
In Wheelock et at I, U.S. Pat. No. 3,087,790 granted Apr. 30, 1963 and Wheelock et al II, U.S. Pat. No. 3,260,035, granted July 12, 1966, there is disclosed the decomposition of calcium sulfate, by heating the calcium sulfate with reducing gases to a relatively high temperature to produce sulfur dioxide and lime, cooling and clearing the gas, and converting the sulfur dioxide via the normal contact process into sulfuric acid.
In the article "Sulfuric Acid from Anhydrate" by W. Q. Hull et al, Industrial Engineering Chemistry, vol. 49, pp. 1207-1214 (1957), there is discussion relating to the decomposition of calcium sulfate in the presence of solid carbon, such as charcoal or coke in kilns, in the cement-sulfuric acid process. Another approach is disclosed in Gorin, U.S. Pat. No. 3,729,551, granted Apr. 24, 1973 wherein calcium sulfate is converted to lime in two stages, the first stage is a separate reduction stage wherein a fluidized bed of relatively coarse calcium sulfate is converted to calcium sulfide by means of hydrocarbonaceous solids and air in a bed, and the second, is a separate oxidation stage wherein CaS from the first stage is converted to CaO and SO.sub.2.
It is also known in the prior art that alkaline earth metal oxides may be used to remove sulfur dioxide from stack gases. In Thornton, U.S. Pat. No. 3,475,121, granted Oct. 28, 1969, there is described a process wherein the alkaline earth oxide reacts with the SO.sub.2 to form the corresponding sulfite. The sulfite is separated and then decomposed to recover SO.sub.2 and the metal oxide, which metal oxide is then recycled to the adsorption step.
Specifically as related to the fluidized-bed combustion of coal, it was also desirable to remove the evolving sulfur oxides. Limestone was suggested as a possible material for removal of sulfur oxides. Attendant high limestone requirements, to wit, Ca/S ratios in excess of two, prompted efforts towards the development of a regeneration system. Certain regeneration systems were based on the aforesaid Wheelock et al I and II reductive decomposition. Such systems required high regeneration temperatures (1100.degree. C.), relatively high quantities of the carbon gases, with low concentrations of product SO.sub.2.
There is now provided by the present invention a process for lime regeneration from sulfated limestone based on a solid carbon reaction with the sulfated limestone.
It is therefore a principal object of this invention to provide a process for regenerating lime for a fluidized-bed combustion system, which regeneration process is based on a carbon-sulfated lime reaction.
It is another object of this invention to provide the process as immediately aforesaid wherein the ash recovered from the fluidized-bed reaction is utilized in the lime regenerative process.
It is another object of this invention to provide a process for the regeneration of lime from sulfated lime in essentially a single kiln operation.
It is another object of this invention to provide a process for the regeneration of lime wherein the unburnt carbon in the FBC fly ash may be effectively utilized and the need for a separate carbon burn-up cell as described in the U.S. Pat. No. 3,508,506 is reduced or eliminated.
It is another object of this invention to provide a process for the regeneration of lime in a kiln wherein the lime suffers less attrition than lime regenerated in a second fluidized-bed reactor.
It is still another object of this invention to provide a process for the regeneration of lime in a fluidized-bed combustion system, wherein the temperatures of regeneration are lower than heretofore.
It is still another object of this invention to provide a process for the regeneration of lime in a fluidized-bed combustion system wherein the regenerated lime has a higher reactivity than lime regenerated at the higher temperatures used heretofore.
It is still a further object of this invention to provide a process for the regeneration of lime in a fluidized-bed combustion system, wherein the sulfur dioxide gases emanating from the regenerator kiln, are in relatively high concentrations.
It is also an object of this invention to provide a process for the regeneration of lime in a fluidized-bed coal combustion system which is ecologically sound while being commercially feasible.